A quick dive into the protist world (Part II): Plastids

Psi Wavefunction is a graduate student working with protists (the 'other' eukaryotes) at Dalhousie University in Halifax, Nova Scotia, and now blogs about protists and evolution at Skeptic Wonder. Follow on Twitter @Ocelloid.

Psi Wavefunction is a graduate student working with protists (the 'other' eukaryotes) at Dalhousie University in Halifax, Nova Scotia, and now blogs about protists and evolution at Skeptic Wonder. Follow on Twitter @Ocelloid.

In the previous post of this series (way too long ago…), we went on a little diving adventureinto the microscopic world with our ocelloid-bearing Nematodinium, starting off with giant kelp forests and gradually zooming into the critters living on the blade surfaces and wading deep into the molecular world of genomes — barely scratching the surface, of course. In this installment, we’ll look a little more at how these critters interact with each other — and with bacteria — sometimes with lasting consequences. Perhaps among the more familiar of these relationships is that of eukaryotes with their domesticated cyanobacterium — the plastid (commonly referred to as a chloroplast in plants). Among other metabolic roles, the plastid most importantly converts carbon dioxide (in case of plants, in the form of an atmospheric gas) to a carbon source the organisms can actually use — sugars. The plastids need not be our familiar shades of green, and often lining a rocky sea shore is a colourful spectrum of seaweeds. Perhaps most prominent are the red algae, which use some different pigments and don’t look any shade of green.

Looking up at towering kelp forest above us, we see photosynthetic organisms of yet another colour, ones that have a rather elaborate story to tell. It is not particularly obvious that the reason kelps can harness cyanobacterial phytosynthetic might lies in ancient relatives of the lowly red crust surrounding their holdfasts (‘roots’). Millions of years ago, a predatory eukaryote chomped on a single-celled red alga and kept it (I’m mostly lying — but bear with me, for the sake of brevity!) — this red alga lost its nucleus and became a secondary plastid endosymbiont. What followed is an explosion of algal diversity, encompassing organisms as disparate as scale-forming haptophytes, dinoflagellates, glass-encased diatoms, apicomplexan parasites like the malaria pathogen, and our giant multicellular kelps. Even seemingly definitively un-algal things like ciliates and fungal-like oomycetes appear to have been photosynthetic in their distant past. Collectively, the plastids of these organisms are referred to as ‘brown’, owing to their often brownish appearance — the kelps as an obvious showcase.

The tale of eukaryotic plastids (‘chloroplasts’). Not the pattern of engulfment and establishment of the plastid, followed be loss of various features like membranes. (Keeling 2004 Am J Bot)

One group of these brown algae, although perhaps brown from a separate red algal plastid symbiosis event, has a remnant nucleus of the ex-red algal slave. These flagellates, called cryptomonads, have an extremely shrunken second nucleus (a ‘nucleomorph’, even) associated with the plastid — these nucleomorph genomes are famous among the molecular evolution folks for their extremely compact, streamlined genomes, with very few, short introns and small intergenic regions. In other words, whatever factors were responsible for causing a shrinkage of the nucleomorph genome led to a massive purging of junk DNA sequences that most eukaryotic genomes are festering with.

Like most good things, plastids can also be stolen. And repackaged. And sometimes stolen again. We watch a herd of cryptomonads casually swirling about, soaking up the final rays of daylight. Suddenly, into the midst of the frolicking algae jumps a sizeable ciliate, and devours a cryptomonad. It then proceeds to separate out the plastid with its nucleomorph and other associates, as well as the cryptomonad nucleus — and packages them together into a vesicle! Presumably, in this manner, the ciliate prolongs the use of its stolen photosynthetic equipment by keeping its original life support nearby.

Those who partake in the life of crime must seldom let their guard down, however — for they have plenty of colleagues hell-bent on acquiring a taste of their freshly stolen wealth. We may gasp as Dinophysis, a dinoflagellate, suddenly descends upon the ciliate thief and stabs it with a straw. You can see individual stolen plastids, ex-cryptomonads, being sucked up through this straw and into the dinoflagellate. Not only is this dino itself a thief, it can’t do its criminals activities independently — apparently, the cryptomonads are toxic to Dinophysis unless tamed by the ciliate. Alterntively, cryptomonads are quite fast, and perhaps the sluggish Dinophysis simply cannot catch up to them — the ciliate, Myrionecta, has been known to jump.

It may be particularly perplexing that Dinophysis generally seems to come with its own plastids. For a long time, people have attempted to culture Dinophysis like its fellow photosynthetic dino brethren, assuming it should be perfectly capable of living off sunlight and a nutritious medium. For many years this elusive (and often beautiful!) organism could not be cultured. The discovery of its strange association with Myrionecta and cryptomonads enabled them to finally be cultured, but it remains an open question whether they do have their own plastids and merely supplement them with theft, or subsist entire on stolen goods. This story is discussed in somewhat greater detail in this post on the other blog, and remains an active research topic.

Photosynthesis is not the only wealth sought after in symbiotic relationships, and our story will continue next time as we meet more bacterial denizens of protistan cells, following a brief encounter with a tertiary plastid endosymbiont, just to drive the point home that symbiosis, while not necessarily by any means a ‘fundamental driving force’ in evolution, is a fairly common and important phenomenon. It’s just too easy, one you maintain a constant partner within yourself, to become dependent upon it.

About the Author: Psi Wavefunction is a graduate student working with protists (the 'other' eukaryotes) at Dalhousie University in Halifax, Nova Scotia, and now blogs about protists and evolution at Skeptic Wonder. Follow on Twitter @Ocelloid.